System and method for managing DC link switching harmonics

ABSTRACT

A distributed motor drive system includes a power management module and multiple inverter modules integrated with the motors and located on a machine or process remote from the power management module. The power management module distributes DC voltage and command signals to each of the inverters, where the DC voltage is distributed between modules via a DC link cable. The integrated inverters execute switching routines to convert the DC voltage to an AC voltage suitable for controlling the motor. Each of the power management module and the inverters includes a portion of the DC bus. The current on the DC link cable is monitored and general bus utilization as well as overload conditions are reported.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. Ser.No. 13/295,690, filed Nov. 14, 2011, the entire contents of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates generally to a distributedmotor drive system and, more specifically, to a system and method formanaging harmonic currents present on a direct current (DC) link in adistributed motor drive system.

Alternating current (AC) motors receive an AC voltage at the stator ofthe motor. The speed and torque of the motor are controlled by varyingthe amplitude and frequency of this AC voltage applied to the stator. Inorder to provide varying AC voltage waveforms, a motor controllerrapidly switches solid state devices on and off at a predeterminedswitching frequency and, thereby, alternately connects or disconnectsthe terminals of the motor to a DC voltage. By varying the durationduring each switching period for which the terminal of the motor isconnected to the DC voltage, the magnitude of the output voltage isvaried. The motor controller utilizes modulation techniques such aspulse width modulation (PWM) to control the switching and to synthesizewaveforms having desired amplitudes and frequencies.

Industrial applications which utilize servo motors, such as a processline with multiple stations, a machining center, or an industrialrobotic arm, often have multiple axes of control. Each axis requires amotor and a controller to regulate, for example, the speed, position, ortorque of the motor. The motors are necessarily positioned along theprocess line or about the machine as needed to actuate a specificmotion. The motor controllers are typically located within one or moreenclosures at a common location. However, developments in the powerelectronic devices used to control the motor have reduced the size ofthe components. This reduction in size of the power electronic devicesalong with a desire to reduce the size of the control enclosures haveled to placing at least a portion of the motor controller electronics onthe motor itself.

Such integrated motor and motor controller systems have not been fullymet without incurring various disadvantages. According to one suchsystem, the entire motor controller has been mounted on the motor.However, even with the reduction in size of the power electroniccomponents, including the converter, inverter, and DC bus capacitance onthe motor still requires a considerable amount of space, especially asthe current rating of the motor increase. Further, the heat generated byboth the converter and inverter power electronics must be dissipated atthe motor.

According to another system for integrating the motor and motorcontroller, only the inverter section of the motor controller is mountedon the motor. The rectifier section and DC bus capacitance remains inthe control enclosure. Although this system reduces the space requiredon the motor and also reduces the amount of heat that must be dissipatedat the motor, another drawback arises. The inverter section receives aDC voltage via a DC link cable from a DC bus output of the rectifiersection in the control enclosure. Although a small amount of capacitancemay be connected across the DC link at the inverter section, modulationof the solid state devices in the inverter produces harmonic currents atmultiples of the inverter switching frequency, which are, subsequently,conducted on the DC link between the inverter section and the controlenclosure.

Another disadvantage in such a system is that the DC link cableextending between the control enclosure and the inverter section canestablish a resonant frequency as a function of the length of the DClink cable. If the length of the DC link cable is selected such that theresonant frequency is close to the switching frequency of the inverter,the harmonic current on the DC link may be amplified. To avoidamplification of the harmonic current specific lengths of the DC linkcable may be required. Alternately, to compensate for the harmoniccontent the size of the conductor is increased or the current rating ofthe conductor is reduced. Each of these options introduces anundesirable cost or limitation in the system.

BRIEF DESCRIPTION OF THE INVENTION

The subject matter disclosed herein describes a distributed motor drivesystem which includes a power management module and multiple invertermodules integrated with the motors and located on a machine or processremote from the power management module. The power management moduledistributes DC voltage and command signals to each of the invertermodules. The DC voltage is distributed between modules via a DC linkcable. The integrated inverters execute switching routines to convertthe DC voltage to an AC voltage suitable for controlling the motor. Thepower management module and each of the inverter modules includes aportion of the DC bus capacitance. The current on the DC link cable ismonitored and general bus utilization as well as overload conditions arereported.

According to a first embodiment of the invention, a method for managinga current on a conductor linking a DC bus of a power management moduleand a DC bus of at least one remote inverter module is disclosed. Thecurrent includes at least one harmonic component and is sampled at afirst sampling rate with a current sensor. A root mean square (rms)value of the current sampled at the first sampling rate is determinedand compared against a first threshold. A first output signal isgenerated if the rms value of the current sampled at the first samplingrate is greater than the first threshold and is reset if the rms valueof the current sampled at the first sampling rate is less than the firstthreshold. The amplitude of the current present on the DC bus of thepower management module is sampled at a second sampling rate slower thanthe first sampling rate. A rms value of the current sampled at thesecond sampling rate is determined, and a second output signal,corresponding to the utilization rate of the DC bus of the powermanagement module as a function of the rms value of the current sampledat the second sampling rate, is generated.

According to another embodiment of the invention, a method for managingat least one harmonic component of a current on a DC bus of a powermanagement module connected to at least one remote inverter modulesamples at a first sampling rate an input signal corresponding to anamplitude of the current present on the DC bus of the power managementmodule. A root mean square (rms) value of the current sampled at thefirst sampling rate is determined. The rms value of the current sampledat the first sampling rate is downsampled to a second sampling rateslower than the first sampling rate, and a rms value of the current isdetermined at the second sampling rate. A utilization rate of the DC busof the power management module is determined as a function of the rmsvalue of the current sampled at the second sampling rate.

According to still another embodiment of the invention, a system forcontrolling a plurality of motors distributed about an industrialmachine or process includes a rectifier section configured to receive anAC voltage input and to convert the AC voltage input to a DC voltage. Alocal DC bus receives the DC voltage from the converter section, and afirst capacitance is connected across the local DC bus. A plurality ofmotor control modules, are each mounted to one of the motors and furtherinclude a remote DC bus connected to the local DC bus via at least oneDC link cable, a second capacitance smaller than the first capacitanceconnected across the remote DC bus, and an inverter section including aplurality of switches converting the DC voltage to an AC voltagesuitable for controlling the motor. The system also includes aninductance connected in series between the local DC bus and the remoteDC bus of a first of the motor control modules, and a current sensingdevice, sampling the current present on the local DC bus at a firstsampling frequency and generating a corresponding signal. A processorconfigured to execute a stored program executes to compare the signalfrom the current sensing device against a first threshold.

These and other advantages and features of the invention will becomeapparent to those skilled in the art from the detailed description andthe accompanying drawings. It should be understood, however, that thedetailed description and accompanying drawings, while indicatingpreferred embodiments of the present invention, are given by way ofillustration and not of limitation. Many changes and modifications maybe made within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments of the subject matter disclosed herein areillustrated in the accompanying drawings in which like referencenumerals represent like parts throughout, and in which:

FIG. 1 is an exemplary distributed motor system according to oneembodiment of the invention;

FIG. 2 is a block diagram representation of an exemplary distributedmotor system according to one embodiment of the invention;

FIG. 3 is a schematic representation of an inverter section from FIG. 2;

FIG. 4 is a graphical representation of a segment of one phase of an ACvoltage output by an inverter section according to a PWM routine;

FIG. 5 is a block diagram representation of one embodiment of a currentmonitoring system according to the present invention;

FIG. 6 is a block diagram representation of the aggregate powercalculation module as shown in FIG. 5;

FIG. 7 is a block diagram representation of the fast rms calculationmodule as shown in FIG. 5; and

FIG. 8 is a block diagram representation of the slow rms calculationmodule as shown in FIG. 5;

In describing the various embodiments of the invention which areillustrated in the drawings, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific terms so selected and it is understood thateach specific term includes all technical equivalents which operate in asimilar manner to accomplish a similar purpose. For example, the word“connected,” “attached,” or terms similar thereto are often used. Theyare not limited to direct connection but include connection throughother elements where such connection is recognized as being equivalentby those skilled in the art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning initially to FIG. 1, a distributed motor control system 10according to one embodiment of the invention is disclosed. An inputvoltage 11 is connected to a converter module 14 via a suitabledisconnect 12, such as a fuse block or a circuit breaker. The inputpower 11 may be any suitable power supply such as three phase or singlephase AC voltage, according to the application requirements. Theconverter module 14 converts the AC voltage into a DC voltage fordistribution by the power management module 30. The converter module 14and power management module 30 are mounted together in a controlenclosure positioned near a machine or industrial process to becontrolled. It is contemplated that the converter module 14 and thepower management module 30 may either be separate modules or integratedas a single module without deviating from the scope of the invention.Further, separate modules may be connected, for example, by electricalconductors or by a backplane connection in a rack style enclosure. Theconnection may be used to transmit both power and control signalsbetween modules. Optionally, a DC voltage may be provided directly tothe power management module 30 via the disconnect 12.

The power management module 30 is connected in series with each powerdelivery module, also referred to herein as an inverter module 50. Acable 18 extends between the power management module 30 and a first ofthe inverter modules 50 as well as between subsequent inverter modules50. The cable 18 includes all necessary connections to transmit, forexample, power, reference commands, and/or network communicationsbetween each series connected inverter module 50. The cable maysimilarly include feedback signals from the inverter module 50 to thepower management module 30. The cable 18 may include multiple conductorscontained within a single jacket and appropriate shielding, separateconductors for each of the electrical connections, or a combinationthereof.

Each power delivery module 50 is mounted to a motor 15. Each motor 15 ismounted remotely from the control enclosure and connected to the machineor industrial process being controlled. The power delivery modules 50include housings connected to the motor housing, for example, by boltsor which are, optionally, integrally formed with the corresponding motorhousing. The voltage output to each motor 15 from the power deliverymodule 50 may be either a single or multi-phase AC output voltageaccording to the requirements of the motor 15. Optionally, each motor 15may include a position sensor 51 such as an encoder or a resolverproviding a signal corresponding to the angular position of the motor15.

Referring next to FIG. 2, the converter module 14 includes a rectifiersection 16, connected in series between the input voltage 11 and a DCbus 13, and a first DC bus capacitor 21 connected across the DC bus 13.It is understood that the DC bus capacitor 21 may be a single capacitoror multiple capacitors connected in parallel, in series, or acombination thereof. The rectifier section 16 may be either passive oractive, where a passive rectifier utilizes electronic devices such asdiodes, which require no control signals, and an active rectifierutilizes electronic devices including, but not limited to, transistors,thyristors, and silicon controlled rectifiers, which receive switchingsignals to turn on and/or off. The converter module 14 also includes aprocessor 26 and a memory device 27. It is contemplated that theprocessor 26 and memory device 27 may each be a single electronic deviceor formed from multiple devices. Optionally, the processor 26 and/or thememory device 27 may be integrated on a field programmable array (FPGA)or an application specific integrated circuit (ASIC). The processor 26may send or receive signals to the rectifier section 16 as required bythe application requirements. The processor 26 is also configured tocommunicate with external devices via an industrial network 28,including but not limited to, DeviceNet, ControlNet, or Ethernet/IP andits respective protocol. The processor 26 further communicates withother devices within the motor control system 10 via any suitablecommunications medium 29, such as a backplane connection or anindustrial network, which may further include appropriate networkcabling and routing devices.

The power management module 30 is connected in series with the convertermodule 14. Electrical connections are established between the DC bus 13of the converter module 14 and a DC bus 33 in the power managementmodule 30 to transfer the DC bus voltage between modules. Theconnections may be made via a backplane connection, a power bus, or viaa DC link cable 23. Alternately, if the two modules are integrated, thetwo DC busses, 13 and 33, may be combined into a single DC bus. The DCbus 33 includes a first voltage rail 32 and a second voltage rail 34.Each of the voltage rails, 32 or 34, are configured to conduct a DCvoltage having a desired potential, according to applicationrequirements. According to one embodiment of the invention, the firstvoltage rail 32 may have a DC voltage at a positive potential and thesecond voltage mil 34 may have a DC voltage at ground potential.Optionally the first voltage rail 32 may have a DC voltage at groundpotential and the second voltage rail 34 may have a DC voltage at anegative potential. According to still another embodiment of theinvention, the first voltage rail 32 may have a first DC voltage at apositive potential with respect to the ground potential and the secondvoltage rail 34 may have a second DC voltage at a negative potentialwith respect to the ground potential. The resulting DC voltage potentialbetween the two voltage rails, 32 and 34, is the difference between thepotential present on the first rail 32 and the second rail 34.

The power management module 30 further includes a processor 40 and amemory device 42. It is contemplated that the processor 40 and memorydevice 42 may each be a single electronic device or formed from multipledevices. Optionally, the processor 40 and/or the memory device 42 may beintegrated on a field programmable array (FPGA) or an applicationspecific integrated circuit (ASIC). The processor 40 in the powermanagement module 30 is in communication with the processor 26 in theconverter module 14 via the communications medium 29. The communicationsmedium 29 may be integrated into a backplane connection, integrated withthe DC link cable 23 as a single cable, or provided as a separatenetwork cable. The power management module 30 also includes at least onesensor 35 connected to the DC bus 33 and in communication with theprocessor 40. Each sensor 35 provides a signal to the processorcorresponding to an operating condition, such as the amplitude of thevoltage or current present on the DC bus 33.

A DC link inductance 20 is mounted within the power management module 30and connected in series with the DC link cable 23 to a power deliverymodule 50. Optionally, the inductance 20 may be mounted external to thepower management module 30. According to one embodiment of theinvention, the DC link inductance includes a first inductor 22 connectedin series between the positive voltage rails, 32 and 52, of the powermanagement module 30 and the power delivery module 50, respectively, anda second inductor 24 connected in series between the negative voltagerails, 34 and 54, of the power management module 30 and the powerdelivery module 50, respectively.

According to one embodiment of the invention, the DC bus 53 of the firstpower delivery module 50 is connected in series with the DC bus 33 ofthe power management module 30 and the DC bus 53 of subsequent powerdelivery modules 50 are connected in series with the DC bus 53 of thepreceding power delivery module 50. Electrical connections areestablished between the DC bus 53 of each power delivery module 50 andits preceding module via a DC link cable 23 to transfer the DC busvoltage between modules. Similar to the DC bus 33 in the powermanagement module 30, the DC bus 53 in the power delivery module 50includes a first voltage rail 52 and a second voltage rail 54. Each ofthe voltage rails, 52 or 54, are configured such that they may conduct aDC voltage having the same potential as the voltage rails, 32 or 34, inthe power management module 30.

The power delivery module 50 further includes a processor 62 and amemory device 64. It is contemplated that the processor 62 and memorydevice 64 may each be a single electronic device or formed from multipledevices. Optionally, the processor 62 and/or the memory device 64 may beintegrated on a field programmable array (FPGA) or an applicationspecific integrated circuit (ASIC). The processor 62 in each powerdelivery module 50 is in communication with preceding module via thecommunications medium 29. The communications medium 29 and the DC linkcable 23 define, at least in part, the cable 18 extending betweenmodules. The power delivery module 50 also includes at least one sensor55 connected to the DC bus 53 and in communication with the processor40. Each sensor 55 provides a signal to the processor corresponding toan operating condition, such as the amplitude of the voltage or currentpresent on the DC bus 33. At least one additional sensor 57 is connectedto the output of the inverter section 60 and provides a signal to theprocessor 62 corresponding to the current in one of the phases of the ACoutput voltage to the motor 15.

A DC bus capacitor 56 is connected between the positive and negativerails, 52 and 54, to reduce the magnitude of the ripple voltage presenton the DC bus 53. It is understood that the DC bus capacitor 56 may be asingle capacitor or multiple capacitors connected in parallel, inseries, or a combination thereof. The magnitude of the voltage potentialbetween the two voltage rails, 52 and 54, is generally about equal tothe magnitude of the peak of the AC input voltage 11. The DC voltage onthe DC bus 53 is converted to an AC voltage by an inverter section, 60.According to one embodiment of the invention, the inverter section 60converts the DC voltage to a three-phase output voltage available at anoutput terminal 58 connected to the motor 15. The inverter section 60includes multiple switches 71 which selectively connect one of theoutput phases to either the positive voltage rail 52 or the negativevoltage rail 54. Referring also to FIG. 3, each switch 71 may include atransistor 72 and a diode 73 connected in parallel to the transistor 72.Each switch 71 receives a switching signal 75 to enable or disableconduction through the transistor 72 to selectively connect each phaseof the output terminal 58 to either the positive voltage rail 52 or thenegative voltage rail 54 of the DC bus 53.

In operation, the converter module 14 receives an AC input voltage 11and converts it to a DC voltage with the rectifier section 16. The ACinput voltage 11 may be either a three phase or a single phase ACvoltage. If the rectifier section 16 is an active rectifier, theprocessor 26 will receive signals from the active rectifiercorresponding to, for example, amplitudes of the voltage and current onthe AC input and/or the DC output. The processor 26 then executes aprogram stored in memory 27 to generate switching signals to activateand/or deactivate the switches in the active rectifier, where theprogram includes a series of instructions executable on the processor26. In addition, the switching signals may be generated such that poweris transferred in either direction between the AC input and the DCoutput. Whether there is a passive rectifier or an active rectifier, theDC bus capacitor 21 connected across the DC bus 13 reduces the ripplevoltage resulting from the voltage conversion. The DC voltage from theDC bus 13 of the converter module 14 is then provided to the DC bus 33of the power management module 30.

The processor 26 of the converter module 14 may further be configured tocommunicate with other external devices via the industrial network 28.The processor 26 may receive command signals from a user interface orfrom a control program executing, for example, on a programmable logiccontroller. The command signals may include, but are not limited to,speed, torque, or position commands used to control the rotation of eachmotor 15 in the distributed control system 10. The processor 26 mayeither pass the commands directly or execute a stored program tointerpret the commands and subsequently transmit the commands to eachinverter module 50. The processor 26 communicates with the processors,40 or 62, of the power management module 30 and the inverter modules 50,directly or via a daisy chain topology and suitable communications media29. Further, the processor 26 may either communicate using the samenetwork protocol with which it received the commands via the industrialnetwork 28 or convert the commands to a second protocol for transmissionto subsequent modules, 30 or 50.

The power management module 30 transfers the DC voltage and the controlsignals received from the converter module 14 to each power deliverymodule 50. The series connection of the DC bus 33 of the powermanagement module 30 between the DC bus 13 of the converter module 14and the DC bus 53 of the first power delivery, or inverter, module 50establishes an electrical conduction path for the DC voltage between theconverter module 14 and the inverter module 50. The DC bus capacitor 36in the power management module 30 further reduces the ripple voltage onthe DC bus resulting from voltage conversion. Optionally, the powermanagement module 30 and the converter module 14 may be integrated as asingle unit, resulting in the combination of DC busses 13 and 33 into asingle DC bus and the combination of DC bus capacitors, 21 and 36, intoa single capacitance. The processor 40 receives a signal from at leastone sensor 35 corresponding to the amplitude of the current and/orvoltage present on the DC bus 33. Further, the processor 40 samples theamplitude of the current from the sensor 35 at a fast enough rate tomonitor the amplitude of harmonic currents present on the DC bus 33.

Bach power delivery module 50 converts the DC voltage from the DC bus 53to an AC voltage suitable to control operation of the motor 15 on whichit is mounted. The processor 62 executes a program stored on a memorydevice 64. The processor 62 receives a reference signal via thecommunications medium 29 identifying the desired operation of the motor15. The program includes a control module configured to control themotor 15 responsive to the reference signal and to feedback signals,including but not limited to signals from the voltage sensor 55, thecurrent sensors 57, and the position sensor 51. The control modulegenerates a desired voltage reference signal 104, see also FIG. 4. Theprogram further includes a switching module using, for example, pulsewidth modulation (PWM) to generate switching signals 75 to control theswitches 71 responsive to the desired voltage reference signal 104.

Referring next to FIG. 4, a segment of one phase of an AC voltage outputaccording to an exemplary sine-triangle PWM modulation technique 100 isillustrated. In the sine-triangle PWM modulation technique 100, atriangular waveform 102 is compared to the voltage reference 104 togenerate switching signals 75. The switching signals 75 control theswitches 71 that selectively connect or disconnect each phase of theoutput terminal 58 to either the positive voltage rail 52 or thenegative voltage rail 54. One period of the triangular waveform 102 isdefined by the switching period 106 of the PWM routine. During theswitching period 106, if the voltage reference 104 is greater than thetriangular waveform 102, the switching signal 75 is set high and, if thevoltage reference 104 is less than the triangular waveform 102, theswitching signal 75 is set low. The resulting output voltage 108 can berepresented by a stepped waveform where the magnitude of the steppedwaveform during each period 106 is the average value of the outputvoltage 108 during that period 106. The average value is determined bymultiplying the magnitude of the DC voltage present on the DC bus 53 bythe percentage of the period 106 that the switching signal 75 is sethigh. As the switching period 106 of the PWM routine decreases, thestepped output voltage 108 more accurately corresponds to the voltagereference 104. It is contemplated that other modulation techniques, aswould be known to one skilled in the art, may also be used to generatethe output voltage, such as space-vector or multi-level switching.Further, the modulation techniques may be implemented by comparinganalog signals, as shown in FIG. 4; digital signals, such as a registerbeing incremented up and down; or a combination thereof.

The alternating connection and disconnection of the switches 71 to theDC bus 53 creates a ripple or fluctuation in the amplitude of thecurrent on the DC bus 53. The amplitude and frequency of the ripple maybe affected by many factors, including but not limited to the switchingfrequency and the modulation technique utilized. The modulation of theswitches 71 may generate harmonic currents on the DC bus 53, forexample, at the frequency, or multiples thereof, of the switchingfrequency. In addition, the switching routine may introduce harmoniccurrents at still other frequencies. Although the bus capacitor 56 onthe power delivery module 50 helps reduce the ripple and the resultingharmonic currents present on the DC bus 53 in the power delivery module50, the physical size restraints from mounting the power delivery module50 on the motor 15 restrict the amount of capacitance that may beincluded in the power delivery module 50, which, in turn, restricts theeffectiveness of the capacitance 56 in reducing the harmonic currents.

Although the DC bus capacitor 56 of the power delivery module 50 isconnected in parallel with the DC bus capacitors, 21 and 36, of theconverter and power management modules, 14 and 30 respectively,increasing the system capacitance, the DC link cable 23 connecting theDC bus 53 of the power delivery module 50 to the DC bus 33 of the powermanagement module 30 may amplify any harmonic current generated by theswitches 71 in the power delivery module 50. The DC link cable 23introduces reactive components, for example a cable inductance, that area function of the length of the DC link cable 23. Consequently, theamount the current is amplified is a function of the frequency of theharmonic current and the length of the DC link cable 23. Further, thedistance the motors 15 and the power delivery modules 50 are locatedfrom the central enclosure may vary from one or two meters up tohundreds of meters.

The inductance 20 mounted in series with the DC link cable 23 isconfigured to mitigate the harmonic currents present in the distributedmotor control system 10. The magnitude of the inductance 20 is selectedto attenuate harmonic content at the lowest frequency of harmoniccurrent expected in the distributed motor control system 10. Aspreviously discussed, the harmonic currents on the DC link cable 23 area function of the switching frequency, the modulation technique, thelength of the DC link cable 23, and the size of the capacitance 56present in the power delivery module 50. According to one exemplaryembodiment of the invention, a modulation technique is selected which,for example, results in 2nd and 4th harmonic content being present onthe DC link cable 23. If the switching frequency is selected at 2 kHz,the resulting harmonic currents would have frequencies at 4 and 8 kHz.The magnitude of the inductance is selected such that the resulting DClink impedance attenuates the 4 kHz harmonic current to an acceptablelevel. According to another exemplary embodiment of the invention, theharmonic currents generated are a function of the modulation techniqueimplemented. Again, the magnitude of the inductance is selected suchthat the resulting DC link impedance attenuates the harmonics generatedfrom the modulation technique to an acceptable level. Preferably, theattenuation results in about a 0 dB gain of the harmonic currents.

Although attenuated, the harmonic content on the DC link cable 23 is noteliminated. Referring to FIGS. 5-8, one embodiment of monitoring andmanaging the harmonic currents resulting from the switching of theinverter modules 50 is illustrated. The processor 40 in the powermanagement module 30 receives an input signal corresponding to theamplitude of the DC link current 122. Optionally, another processor inthe motor control system 10, or a processor in an external processingdevice, including but not limited to a programmable logic controller(PLC) or a remote computer, may receive the signal. The input signal issampled at a rate fast enough to measure the harmonic content present.The sampling frequency may be, for example, an order of magnitudegreater than the highest expected harmonic current. Thus, the processor40 is configured to read the signal from the sensor 35 corresponding tothe amplitude of the current on the DC bus 33 at a rate fast enough tomonitor the harmonic current present.

A calibration module 124 applies an offset and scaling factor as neededaccording to the system requirements. The calibration module 124 may beimplemented by a stored program executing on the processor 40 to add anoffset to a digital value corresponding to the DC link current 122 or,optionally, may be implemented via an analog circuit which offsets thesignal corresponding to the DC link current 122 prior to inputting thesignal to the processor 40.

The calibrated current signal 125 is used by an aggregate powercalculation module 130 to determine the power consumed by the chain ofpower delivery modules 50 and motors 15 connected in series with thepower management module 30. The calibrated current signal 125 is firstsent through a low pass filter module 132 to eliminate the harmoniccontent. The bandwidth of the filter is preferably selected to output asignal corresponding to an average value of the DC link current having aresponse time comparable to the DC link voltage. The filtered current ispassed through a down sampling block 134 such that the new frequency ofthe sampled current corresponds to the frequency at which the voltage onthe DC bus 33 is sampled. The down sampling block 134 selects every nthdata point from data sampled at a first frequency to obtain data at asecond, lower frequency, where the value of “n” is selected according tothe desired ratio between the original sampling frequency and thereduced sampling frequency. The down sampled current and the sampled DCvoltage are passed through a multiplying block 136, which outputs theaggregate power consumed by the chain of power delivery modules 50. Theaggregate power is finally passed through an anti-aliasing filter 138.The bandwidth of the anti-aliasing filter is selected as a function ofthe number and of the sampling frequency of the power delivery modules50 connected to the DC bus 33. The calculated value of the aggregatepower may be provided as an output 140, for example, to a display formonitoring by an operator. Optionally, the aggregate power may be storedlocally or remotely in a fixed or removable memory device or provided asan output 140 to another device for display or storage.

The calibrated current signal 125 is also used by a first filter module,for example, a fast root mean square (rms) calculation module 150 todetermine a time-averaged current signal which includes both the realand harmonic current components. According to one embodiment of theinvention, the calibrated current signal 125 enters a multiplying block152 which first multiplies the signal against itself, or squares thesignal. The squared signal is passed through a series of delay blocks154 and summing junctions 156 which form, for example, a 100-tap finiteimpulse response (FIR) filter. Optionally, other lengths of the FIRfilter may be selected. The passband of the filter is selected such thatthe expected harmonic content is included in the pins calculation. Theoutput of the FIR filter enters a gain block 158 to divide the output bythe number of taps in the filter, resulting in the mean of the squaredcurrent. The mean of the squared current enters a square root block 160which outputs the rms value of the sampled current. The fast rmscalculation module 150 next passes the rms value through ananti-aliasing filter 162 and a down sampling block 164 and generatesoutput 166 for subsequent calculations. It is contemplated that firstfilter module may be implemented using other suitable filters accordingto the application requirements.

For example, the output 166 of the fast rms calculation module 150 isprovided as an input to a fast overload detection module 170 todetermine whether the magnitude of the combined harmonic currents and DCcurrent present on the DC link cable 23 exceeds a predeterminedthreshold. The predetermined threshold is selected according to theapplication requirements and is a function of, for example, the size ofthe conductors in the DC link cable 23 or applicable industrialstandards. If the total current on the DC link cable 23 exceeds thepredetermined threshold, the processor 40 generates an output signal,which may be used, for example, to provide a visual or audio alert to anoperator or to generate a message to be transmitted to anotherprocessor.

The output 166 of the fast rms calculation module 150 is also providedas an input to a second filter module, for example, a slow rmscalculation module 190. The input 166 is first passed through a low passfilter block 192 to filter out the harmonic content. The filteredcontent is subsequently down sampled 194 such that a slower processingtask may operate on the sampled current. This filtered currentcorresponds to an average value of the current present on the DC bus 33.Similar to the fast rms calculation module 150, the filtered currentsignal enters a multiplying block 196 which first multiplies the signalagainst itself, or squares the signal. The squared signal is passedthrough a series of delay blocks 198 and summing junctions 200 whichform, for example, a 100-tap finite impulse response (FIR) filter.Optionally, other lengths of the FIR filter may be selected. The outputof the FIR filter enters a gain block 202 to divide the output by thenumber of taps in the filter, resulting in the mean of the squaredcurrent. The mean of the squared current enters a square root block 204which outputs the rms value of the sampled current. The slow rmscalculation module 190 next passes the rms value through ananti-aliasing filter 206 and generates output 208 for subsequentcalculations. It is contemplated that second filter module may beimplemented using other suitable filters according to the applicationrequirements.

For example, the output 208 of the slow rms calculation module 190 isprovided as an input to a bus utilization module 210 which monitors therms current present on the DC bus 33. The rms current may be provided asan output, for example, to a display for monitoring by an operator.Optionally, the rms current may be stored locally or remotely in a fixedor removable memory device or provided as a signal to another device fordisplay or storage. The rms current may also be compared to apredetermined threshold. The predetermined threshold is selectedaccording to the application requirements and is a function of forexample, the size of the conductors in the DC link cable 23 orapplicable industrial standards. If the rms current on the DC bus 33exceeds the predetermined threshold, the processor 40 generates anoutput signal, which may be used, for example, to provide a visual oraudio alert to an operator or to generate a message to be transmitted toanother processor.

The output 208 of the slow rms calculation module 190 may also beutilized to select the DC link inductance 20 in the distributed motorcontrol system 10. A remote processor, such as an industrial controlleror separate computer, may execute a program to simulate an input signalcorresponding to the amplitude of the current present on the DC bus ofthe power management module. The simulated current signal includes theharmonic content expected within the system and is input to a model ofthe distributed motor control system 10 which simulates the dampingcharacteristics of the DC link inductance 20. The output of the systemmodel is then provided as the calibrated current signal 125 input to theblocks monitoring and managing the harmonic current. The output 208 ofthe slow rms calculation module 190 is then monitored and the value ofthe DC link inductance 20 within the system model is varied until theoutput 208, which corresponds to the bus utilization of the distributedmotor control system 10 is below a predetermined threshold. Thus, adesired value of the DC link inductance 20 for use in the system may bedetermined.

It should be understood that the invention is not limited in itsapplication to the details of construction and arrangements of thecomponents set forth herein. The invention is capable of otherembodiments and of being practiced or carried out in various ways.Variations and modifications of the foregoing are within the scope ofthe present invention. It also being understood that the inventiondisclosed and defined herein extends to all alternative combinations oftwo or more of the individual features mentioned or evident from thetext and/or drawings. All of these different combinations constitutevarious alternative aspects of the present invention. The embodimentsdescribed herein explain the best modes known for practicing theinvention and will enable others skilled in the art to utilize theinvention.

We claim:
 1. A method for managing a current on a DC bus shared betweena power management module and at least one remote inverter module,wherein, the current includes at least one harmonic component, themethod comprising the steps of: sampling at a first sampling rate anamplitude of the current present on the DC bus with a current sensor;comparing the amplitude of the current sampled at the first samplingrate against a first threshold; generating a first output signal if theamplitude of the current sampled at the first sampling rate is greaterthan the first threshold and resetting the first output signal if theamplitude of the current sampled at the first sampling rate is less thanthe first threshold; sampling the amplitude of the current present onthe DC bus at a second sampling rate slower than the first samplingrate; and generating a second output signal corresponding to autilization rate of the DC bus as a function of the amplitude of thecurrent sampled at the second sampling rate.
 2. The method of claim 1further comprising the steps of: comparing the amplitude of the currentsampled at the second sampling rate against a second threshold; andgenerating a third output signal if the amplitude of the current sampledat the second sampling rate is greater than the second threshold andresetting the third output signal if the amplitude of the currentsampled at the second sampling rate is less than the second threshold.3. The method of claim 2 wherein at least one of the first output signaland the third output signal generates a fault condition at the remoteinverter module.
 4. The method of claim 1 wherein sampling the amplitudeof the current present on the DC bus at the second sampling rateincludes downsampling of the samples obtained at the first samplingrate.
 5. The method of claim 1 wherein each remote inverter module ismounted to a motor and includes a switching module, the method furthercomprising the initial step of selectively connecting the DC bus at eachinverter module to at least one conductor of a stator present in themotor at a predetermined switching frequency, wherein: at least oneharmonic current is generated as a function of selectively connectingthe DC bus, the at least one harmonic current is transmitted on the DCbus between the at least one remote inverter module and the powermanagement module, and the first sampling rate is greater than thefrequency of the at least one harmonic current.
 6. The method of claim 5wherein: a DC link inductor is connected to the DC bus in series betweenthe power management module and the at least one remote inverter module,and the current sensor samples the current in series between the powermanagement module and the DC link inductor.
 7. The method of claim 6wherein the second sampling rate is less than the frequency of the atleast one harmonic current and greater than a frequency of a servocontrol module configured to execute on a processor in the invertermodule.
 8. The method of claim 1 wherein a plurality of remote invertermodules are electrically connected to receive power from the powermanagement module further comprising the steps of: downsampling thesamples of the amplitude of the current present on the DC bus obtainedat the first sampling rate to a third sampling rate; sampling anamplitude of the voltage present on the DC bus at the third samplingrate; and determining a value of power required by the remote invertermodules as a function of the amplitudes of current and voltage sampledat the third sampling rate.
 9. The method of claim 8 wherein the thirdsampling rate is a function of the number of remote inverter modulesconnected to the power management module.
 10. A method for managing atleast one harmonic component of a current on a DC bus shared between apower management module and at least one remote inverter module, themethod comprising the steps of: sampling at a first sampling rate aninput signal corresponding to an amplitude of the current present on theDC bus; downsampling the amplitude of the current sampled at the firstsampling rate to a second sampling rate slower than the first samplingrate; and generating a utilization rate of the DC bus as a function ofthe amplitude of the current sampled at the second sampling rate. 11.The method of claim 10 further comprising the steps of: comparing theamplitude of the current sampled at the first sampling rate against afirst threshold; generating a first overload signal if the amplitude ofthe current sampled at the first sampling rate is greater than the firstthreshold and resetting the first overload signal if the amplitude ofthe current sampled at the first sampling rate is less than the firstthreshold; comparing the amplitude of the current sampled at the secondsampling rate against a second threshold; and generating a secondoverload signal if the amplitude of the current sampled at the secondsampling rate is greater than the second threshold and resetting thesecond overload signal if the amplitude of the current sampled at thesecond sampling rate is less than the second threshold.
 12. The methodof claim 10 wherein the power management module is in communication witha remote industrial controller and wherein after sampling the inputsignal the method further comprises the step of transmitting the sampledamplitude of the current present on the DC bus to the remote industrialcontroller and wherein the remote industrial controller includes aprocessor configured to execute the remaining steps.
 13. The method ofclaim 10 wherein a remote computing device includes a processorconfigured to execute the steps of the method and the method furthercomprising the initial step of executing a module on the processor togenerate the input signal corresponding to the amplitude of the currentpresent on the DC bus.
 14. The method of claim 13 wherein the moduleexecuting on the processor generates the input signal as a function ofan inductance connected in series between the power management moduleand the remote inverter module.
 15. The method of claim 14 wherein themodule receives the utilization rate as an input and varies theinductance until the utilization rate is below a predeterminedthreshold.
 16. A system for controlling a plurality of motorsdistributed about an industrial machine or process, the systemcomprising: a plurality of motor control modules, wherein each motorcontrol module is mounted to one of the motors, each motor controlmodule further comprising: a remote DC bus configured to receive a DCvoltage, a first capacitance connected across the remote DC bus, and aninverter section including a plurality of switches converting the DCvoltage to an AC voltage suitable for controlling the motor; and a powermanagement module, the power management module further comprising: alocal DC bus electrically connected to the remote DC bus, the local DCbus providing the DC voltage to the remote DC bus, a second capacitanceconnected across the local DC bus, wherein the second capacitance isgreater than the first capacitance, a current sensing device samplingthe current present on the local DC bus at a first sampling frequencyand generating a corresponding signal, and a processor configured toexecute a stored program to compare the signal from the current sensingdevice against a first threshold; and an inductance connected in seriesbetween the local DC bus and the remote DC bus of a first of the motorcontrol modules.
 17. The system of claim 16 wherein a DC link cableincluding a first and a second electrical conductor connect the local DCbus to the remote DC bus and wherein the inductance includes a first anda second inductor connected in series with the first and the secondelectrical conductor, respectively.
 18. The system of claim 16, whereinthe processor of the power management module is further configured todownsample the signal from the current sensing device to a secondsampling frequency slower than the first sampling frequency and togenerate a utilization rate of the local DC bus as a function of thesignal sampled at the second sampling frequency.
 19. The system of claim16, wherein each motor control module further comprises: an outputconfigured to conduct the AC voltage to the motor; and a processorconfigured to execute a series of instructions to generate a pluralityof switching signals at a predetermined frequency, wherein the switchesin the inverter section are controlled by the switching signals toselectively connect the remote DC bus to the output and wherein themagnitude of the inductance is a function of the predeterminedfrequency.
 20. The system of claim 19 wherein the processor of the powermanagement module is further configured to downsample the signal fromthe current sensing device to a second sampling frequency, wherein thesecond sampling rate is less than the frequency of a harmonic currentgenerated as a function of selectively connecting the switches betweenthe remote DC bus and the output of the motor control module and greaterthan the frequency of a servo control module configured to execute onthe processor in the motor control module.